Cobalt can catalyze the formation of acetaldehyde from methanol, carbon monoxide, and hydrogen, a reaction known as methanol reductive carbonylation. For example, it was disclosed by Wender et al., Science, 113, (1951), 206-207, that a cobalt carbonyl catalyst system could be used. However, the product of the disclosed process was primarily ethanol, together with a small amount of acetaldehyde. It was later shown that the addition of iodide to a cobalt-containing catalyst system increased the amount of acetaldehyde produced. Iodide is typically added as a co-catalyst (also commonly referred to as a promoter) to the reaction in a form such as hydrogen iodide (a strong acid), methyl iodide, elemental iodide, or as an iodide salt such as lithium iodide or sodium iodide.
Homologation of methanol to ethanol can be achieved by addition of a hydrogenation catalyst, typically ruthenium based, to a reductive carbonylation system. For example, Mizoroki, et al., Bull. Chem. Soc. Japan, 52, (1979), 479-482, have described a catalyst system containing a cobalt compound, a ruthenium compound and methyl iodide to convert methanol to ethanol with 77% selectivity.
Addition of iodide co-catalysts in these reactions often leads to formation of dimethyl ether as well as free methyl iodide in the crude reductive carbonylation product. Methyl iodide is an undesirable co-product due to the difficulty in separating it from the aldehyde and/or alcohol product as well as its toxicity. Current methanol reductive carbonylation processes carefully balance the amount of iodide containing compounds added to the reaction to obtain optimized reaction rate and conversion while limiting dimethyl ether and methyl iodide formation.
There is a need for an improved catalyst system which will allow reasonable reductive carbonylation reaction rates as well as little to no methyl iodide in the crude reductive carbonylation product. Additionally there is a need to readily influence the relative amounts of aldehyde and/or alcohol produced in a reductive carbonylation reaction to maximize the desired product profile.
There is also a need for an inexpensive catalyst for the reductive carbonylation of alcohol that can replace the typical rhodium catalyst or iridium/ruthenium catalyst while producing a substantially methyl iodide free crude reductive carbonylation product.